In modern gas turbine engines, the rotating components such as the compressor and turbine are designed such that rotor tip clearances are minimised such that gases flowing through the clearance (and thereby not utilised for performing work) is minimised. By minimising rotor tip clearance, engine thermodynamic efficiency is maximised.
However, a small gap must remain in order to prevent excessive tip rubs between the rotor blade and casing. Excessive tip rubs may result in the rotor blade becoming worn, which will in turn shorten the time between overhauls. There is therefore a conflict between the need to minimise the tip clearance for maximum thermodynamic efficiency, and the need to avoid tip rubs in order to extend service life.
Many gas turbine engines utilise “Active Tip Clearance Control” (TCC) arrangements in order to maintain the tip clearance at an optimum value. Tip clearance is difficult to measure directly, and so many prior arrangements use schedules based on a predicted evolution of the turbine to adjust turbine tip clearance. European patent application EP2620601 discloses a TCC arrangement in which the clearance is adjusted over the life of the engine to maintain a target tip clearance. The target tip clearance is constant, and is chosen to minimise fuel burn, while avoiding tip rubs. Similar arrangements are also known for compressor rotors.
The present invention describes a method of controlling a rotor tip clearance in a gas turbine engine and a rotor tip clearance control system which seeks to overcome some or all of the above problems.